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WASHINGTON -- President Obama on Tuesday ordered 350 additional troops be sent to Iraq, as the apparent beheading of a second American journalist raised pressure on the U.S. to take decisive action against the militant group Islamic State and protect Americans remaining in Iraq.

The added deployment, designed to boost security at the U.S. Embassy compound and its support facilities, was not directly linked to what a National Security Council spokeswoman said appeared to be "the brutal murder of an innocent American journalist."

The apparent killing of Steven Joel Sotloff, who also appeared in a video two weeks ago showing the beheading of fellow American journalist James Foley, was presented as a direct challenge to the U.S. government.

"I'm back, Obama," a black-hooded militant says in English on the video, moments before he raises a knife toward Sotloff. "Just as your missiles continue to strike our people, our knife will continue to strike the necks of your people."

U.S. intelligence officials were working as quickly as possible to determine the authenticity of the video, NSC spokeswoman Bernadette Meehan said in a statement. "If genuine, we are appalled ... and we express our deepest condolences to his family and friends."

Sotloff's family, who live in Pinecrest, Fla., did not immediately comment. They were waiting for authentication of the video and were grieving privately, according to a family spokesman, Barak Barfi.

"People's emotional reactions from seeing ISIS carry out these beheadings are what drives the public debate," said Loren Thompson, a military policy analyst for the Lexington Institute in Arlington, Va., using one of several acronyms for the extremist group. "It's forcing Obama to be more decisive and more of a military interventionist than he would probably like to be."

The new troop deployments announced Tuesday bring the U.S. security presence in Iraq to about 820. But the administration stopped short of announcing an expansion of the U.S. campaign against the al-Qaida spinoff group.

"The Department of Defense will continue to plan and prepare further military options should they become necessary, and we will remain ready to protect our diplomats, our citizens, and our interests in Iraq, while we continue to work with the Iraqi government to counter the Islamic State," Pentagon spokesman Rear Adm. John Kirby said.

After the video of Foley's killing appeared Aug. 19, the United States carried out 14 airstrikes against Islamic State targets in Iraq, the second-heaviest bombardment of the air campaign, although there was no indication they were directly related to the journalist's death.

Over the weekend, the Pentagon struck militant strongholds near the town of Amerli, helping to break a siege on the farming community about 100 miles north of Baghdad. U.S. forces continued the campaign Tuesday with the Pentagon confirming another strike near the Mosul dam, where most of the military's 124 attacks have taken place.

In the video made public Tuesday, the militant refers to the U.S. airstrikes, saying, "I'm back because of your arrogant foreign policy toward the Islamic State, because of your insistence on continuing your bombings in Amerli, [unclear] and the Mosul dam, despite our serious warnings."

Sen. Bill Nelson, a Democrat from Sotloff's home state of Florida, said Tuesday that he would introduce legislation giving Obama congressional authority for airstrikes against the militants in Syria, part of a growing chorus in Congress for more decisive action. "We must go after ISIS right away because the U.S. is the only one that can put together a coalition that can stop this group that's intent on barbaric cruelty," said Nelson, a member of the Senate Armed Services Committee.

Florida Gov. Rick Scott, a Republican, also called for action. "I think I can speak for all Floridians and all Americans when I say that the time for a strategy is now, and part of that strategy needs to include destroying them," he said.

The militant Sunni Muslim group has seized large swaths of territory in the north and west of Iraq as well as in neighboring Syria.

In a report released Tuesday, the human rights group Amnesty International said the group has "carried out ethnic cleansing on a historic scale."

Sotloff, 31, a freelance journalist who has written for Time magazine and Foreign Policy, was captured while reporting in Syria in August 2013. His disappearance was not widely known until he appeared at the end of the video of Foley's killing. A masked militant grabbed Sotloff's collar and declared that his life depended on Obama's next decision.

Anthony Cordesman, a former intelligence director at the Pentagon now with the Center for Strategic and International Studies, said killing Sotloff was another way to taunt the West.

"They're saying, 'Yes, America can use its air power, but we have the real power to act here in the region,'" he said.

The release of the video was first reported by the SITE Intelligence Group, which monitors websites used by militants, and posted an edited version. It said it found the recording on a file-sharing site used by Islamic State, which later acknowledged the video on social media.

In the video, a man identifying himself as Sotloff is shown kneeling in a desert dressed in an orange shirt and trousers similar to those worn by detainees at the U.S. military prison at Guantanamo Bay, Cuba.

"I'm sure you know exactly who I am by now, and why I am appearing before you," he says.

"Obama, your foreign policy of intervention in Iraq was supposed to be for the preservation of American lives and interests, so why is it that I'm paying the price of your interference with my life? Am I not an American citizen?"

The militant then delivers his speech to Obama and appears to begin to cut the throat of the captive. The edited video switches to a shot of the captive's head seemingly lying on his body.

SITE said it was the same hooded man who appeared with Foley. Both appeared to be left-handed, but in the new video, the voice was slightly distorted, so it was difficult to determine whether he has the same British accent as the man who killed Foley.

SITE said the life of another captive, identified as British national David Cawthorne Haines, was also threatened.

The release Tuesday came despite pleas from Sotloff's mother, Shirley, in a video that appealed directly to Abu Bakr Baghdadi, the self-appointed caliph of Islamic State, to spare her son.

"We are shocked and deeply saddened by reports of Steven Sotloff's death," Time editor Nancy Gibbs said in a statement. "Steven was a valued contributor to Time and other news organizations, and he gave his life so readers would have access to information from some of the most dangerous places in the world. Our thoughts and prayers are with him and his family."

Hennigan reported from Washington and Zavis from Los Angeles. Times staff writers Michael Muskal in Los Angeles and Lisa Mascaro in Washington and special correspondent Nabih Bulos in Irbil, Iraq, contributed to this report.